A storage virtualization system is disclosed for performing data migration operations among the virtual LU (logical units) defined by the system. In one aspect of the present invention, the selection of a target virtual LU is based on a policy that makes a recommendation relating to the target virtual LU. The recommendation is based on the IO activity of the physical devices which constitute the virtual LUs. In another aspect of the present invention, a replacement policy is recommended to replace a current policy. The replacement policy is based on the IO activity of the physical devices which constitute the virtual LUs.